Seismic exploration today may generally be described as the technological search for hydrocarbon deposits in the Earth's subsurface through indirect observation of the subsurface. One part of seismic exploration is the acquisition of seismic data through what is known as a “seismic survey”. In a seismic survey, seismic signals containing information regarding the subsurface are detected and recorded by seismic receivers. The information is embedded in the seismic signals by the interaction of the seismic signals with the geological features of the subsurface in known, characteristic ways. The information can then be used to reconstruct the subsurface's features. The recorded seismic signals are therefore representative of the tangible, real world subsurface and its features.
Seismic surveys are performed in a variety of natural environments and these natural environments lend themselves to variation in both technique and equipment. So, for example, seismic surveys may be classed as land surveys, marine surveys, and transition zone surveys because they are performed on land, in water, and both on land and in water, respectively. (“Marine” surveys may be performed in any kind of water, whether it be salty, fresh, or brackish.) A simple example of how these environments can affect the design and execution of a seismic survey is the fact that seismic receivers deployed in marine surveys have to deal with currents that are not found in land surveys. Marine seismic surveys therefore must account for these currents where land surveys do not. Other environmental differences yielding such variation abound.
Many seismic surveys employ what are known as “active” seismic sources. Active seismic sources generate the survey's seismic signals and impart them into the subsurface. Sometimes the signals are imparted directly into the subsurface from the ground's surface. In a marine survey, active seismic sources are sometimes positioned on the water's surface, and so the seismic signals travel through the water column before entering the ground instead of being imparted directly into the ground.
One common type of active source is known as a “vibrator” and is used in both land and marine surveys. The term “vibrator” comes from the fact that the energy is “vibratory” in the sense that it is generated through mechanical vibrations. The vibrations can be tailored as to the frequency and the duration of the vibratory seismic signal. (These vibrations are sometimes called “chirps”, “sweeps” or “swept signals”.) However, this is another example of how the technology varies depending on the environment as vibrators used in typical land surveys differ significantly from those used in marine surveys. For example, marine vibrators typically are moving during the survey, which constrains the sweep time lengths possible for a given spatial source interval. This is not true of vibrators in land surveys in which the vibrators are typically stationary in use. Also, vibrators used in land surveys typically constitute of from 1 to 6 mechanical devices whereas in marine surveys the vibrator typically comprises an array of somewhat smaller mechanical devices.
These and other technologies associated with seismic exploration continue to develop responsive to new technical and environmental challenges as well as pressures to reduce cost while increasing accuracy. Such innovation is welcomed by the industry in its efforts to improve the accuracy and efficiency of the seismic exploration process. Accordingly, while acceptable techniques for the conduct of marine seismic surveys are known to the art, the currently disclosed and claimed technique will eventually be well received by those in the art.